1. Field of the Invention
The present invention relates to a plant having enhanced resistance to environmental stress and a method of producing the same.
2. Background Art
Environmental stresses such as drought stress, salt stress and freezing stress significantly restrict growth and life maintenance of plants. Environmental stresses also influence photosynthesis, respiration and the like. Because of this, significant damages such as a reduction in crop-plant productivity are given to agriculture (Nakashima K et al., (2009) Plant Physiol, vol. 149: p. 88-95). Furthermore, environmental stresses also affect carbon fixation ability of plants. Therefore, elucidating the mechanism of resistance to environmental stresses inherent in plants also has a significant meaning for taking various measures against global environmental changes including global warming, and producing transformed plants for increasing food production.
The drought stress affects the leaf size, stem extension, differential proliferation of root, moisture-availability change and the like in plants. Of the various environmental stresses, drought stress gives a lethal damage to life maintenance. To protect plants from the damage, the plants are known to make various physiological and biochemical responses to the stress at a cell level or an organism level. Up to the present, various studies have been made to elucidate and understand the stress resistance mechanism inherent in plants (Nakashima K et al., (2009) Plant Physiol, vol. 149: p. 88-95).
It is known that when a plant senses drought stress, genes for adapting the plant to the environmental change are induced in the plant through a signal transduction pathway primarily based on a transcription factor of DREB-type (Nakashima K et al., (2009) Plant Physiol, vol. 149: p. 88-95; and Seki M et al., (2007) Curr Opin Plant Biol, vol. 10: 296-302). Further, it is known that abscisic acid (ABA), which is a phytohormone synthesized in response to drought stress, induces the expression of a group of ABA-responsive genes, thereby controlling closure of stomas (Nakashima K et al., (2009) Plant Physiol, vol. 149: p. 88-95; Seki M et al., (2007) Curr Opin Plant Biol, vol. 10: 296-302; Yamaguchi-Shinozaki K, Shinozaki K (2006) Annu Rev Plant Biol, vol. 57: p. 781-803; and Wang Y et al., (2004) J Exp Bot, vol. 55: p. 1187-1193). Furthermore, it has been recently reported that the level of histone acetylation on a drought-responsive gene increases in response to drought stress. Thus, control at a chromatin level is also suggested (Kim J M et al., (2008) Plant Cell Physiol, vol. 49: p. 1580-1588; and Kim J M et al., (2010) Plant Cell Environ, vol. 33: p. 604-611). However, a histone modifying enzyme involved in drought-stress resistance has not yet been reported.
Low-temperature stress is one of the factors bringing about a loss of agricultural production and restriction on a region where an agricultural plant can grow. Plants growing in the temperate zone (including Arabidopsis thaliana L.) acquire freezing resistance by passing through a process called cold acclimation which involves a change in gene expression in response to low temperature. It has been so far known that many physiological and molecular changes occur in accordance with cold acclimation. Expression of three transcription factors of CBF/DREB-type is induced rapidly in response to low temperature, thereby inducing the expression of many low-temperature responsive genes such as COR15A gene and RD29A gene. It has been reported that when CBF1/DREB1B gene and CBF3/DREB1A gene are overexpressed in Arabidopsis thaliana L., genes downstream of them are constantly activated, with the result that freezing resistance is acquired. Furthermore, it has been revealed that MYB-type transcription factor ICE1 positively controls induction of expression of CBF3/DREB1A gene through SUMOylation of itself (Lee B H et al., (2005) Plant Cell, vol. 17: p. 3155-3175).
However, it is estimated that low-temperature inducible genes that are independent of these CBF/DREB-type transcription factors account for more than 70% of the genes induced at low temperatures and more than 90% of the genes suppressed at low temperatures. It is suggested that a more complicated low-temperature stress-response control mechanism may be present in parallel to or independently of the known transcription factor-mediated pathway.
Involvement of chromatin control mechanism in controlling low-temperature responsive genes has so far been pointed out or suggested in several reports. A large number of genes that are predicted to influence a chromatin state (e.g., gene having a bromo domain) exhibit low temperature responsiveness (Lee B H et al., (2005) Plant Cell, vol. 17: p. 3155-3175). A transcriptional cofactor ADA2B, which interacts with histone acetyltransferase GCN5, interacts also with a transcription factor CBF1/DREB1B and is involved in expression control of a low-temperature responsive gene (Vlachonasios K E et al., (2003) Plant Cell, vol. 15: p. 626-638; and Mao Y et al., (2006) Biochim Biophys Acta, vol. 1759: p. 69-79). Furthermore, ada2b-1 mutant strain has been reported to exhibit high freezing resistance compared to a wild strain even if it is not subjected to cold acclimation (Vlachonasios K E et al., (2003) Plant Cell, vol. 15: p. 626-638). In addition, it has been shown that histone H4 within a plant body is highly acetylated in a strain having a mutation of a WD40 family gene HOS15, and it has been revealed that a strain having a mutation of the gene exhibits high sensitivity to freezing (Zhu J et al., (2008) Proc Natl Acad Sci USA, vol. 105: p. 4945-4950).